The proposed research will examine the relationship between sociocultural factors and human reproductive physiology. Two recent studies have shown that women who spend time together develop synchronous menstrual cycles. The proposed project will refine the "time spent together" category into specific types of social interaction which occurs between pairs of individuals and among individuals in a group. The population for this study will be about 90 women who belong to a sorority on a large state university campus. The social interaction types will be roommates, pairs of close friends, groups of close friends, and formal groups resulting from the organization of the sorority. Of special interest is the relationship between a pledge and an upperclassman who is her mentor, called a "mother-daughter" relationship. Linear regression-correlations will be used to determine the degree of menstural synchrony for the various types of interacting pairs and groups. The expection is that degree of menstrual synchrony will vary according to type of interaction rather than time spent together. Tests will also be run to determine if those individuals who are socially "dominant" control the menstrual cycles of others. The significance of this research for population change and primate evolution is suggested.